Write chemical formulas for compounds containing each of the following. one nitrogen atom for every three chlorine atoms.

The number goes as the subscript of the atoms in the chemical formula.

So

 is the chemical formula.

Please Note: To find the number of atoms of each element present  

For example contains  

5 × 1 = 5 ,Ca atoms and

5 × 2 = 10, Cl atoms  

If there is a bracket in the chemical formula  

For example we multiply coefficient × subscript × number outside the bracket.......... to find the number of atoms  (Please note: 3 is the coefficient, and if there is no number given then 1 will be the coefficient )

So

3 × 3 = 9 , Ca atoms  

3 × 1 × 2 = 6, P atoms  

3 × 4 × 2 = 24, O atoms are present.

i think   the answer is c hope i'm right